Analyses of neutronic characteristics of STACY heterogeneous core with 1.5-cm-lattice-pitch fuel pins

A series of critical experiments using a heterogeneous core of the Static Experiment Critical Facility (STACY) in the Japan Atomic Energy Research Institute is planning in F.Y. 2003. In the experiment, the core is composed of uranyl nitrate solution (235U enrichment 6 wt%) and 333 pins of uranium dioxide (235U enrichment 5 wt%) loaded in lattice-pitch of 1.5 cm. Prior to the experiment, neutronic characteristics are analyzed to evaluate neutronic safety and criticality limitations of the core. The analyzed items are the parameters on criticality, reactivity and reactor shutdown margins. In the analyses, a Monte Carlo code, MVP, and a neutronics code system, SRAC, have been used with an evaluated nuclear data library, JENDL-3.3. By using the calculated characteristics, simplified equations to interpolate these values and criticality limitations of the core are evaluated. It has been also confirmed that the reactor shutdown margins will comply with safety criteria under all fuel conditions in the experiments. (author)